This invention relates to apparatus for determining the arrival time of alternating signals and, more particularly, to apparatus for determining the arrival time of an ultrasonic signal.
From German Pat. No. 2,322,749 and U.S. Pat. No. 3,780,577 devices are known for the ultrasonic measurement of the flow velocity of fluent media and the sonic velocity in fluent media, in which ultrasonic signals are alternately transmitted upstream and downstream along a measuring path which is provided with two ultrasonic transducers and has at least one component extending in the flow direction. At the instant of transmission, the transducer on the transmission side is momentarily energized and a time generator is simultaneously actuated which delivers a reference signal after the expected transit time. The time generator may include a voltage-controlled oscillator which is operated at a frequency f.sub.1 during downstream measurement and with a frequency f.sub.2 during upstream measurement. A series-connected counter counts a predetermined number of output impulses after the instant of transmission. If the expected transit time is shorter than the actual transit time, the frequency of the oscillator is reduced by means of a regulating circuit; if the expected transit time is longer than the actual transit time, the frequency of the oscillator is increased by means of the regulating circuit. By reason of this adaptation, the oscillator frequencies correspond to the reciprocal values of the downstream and upstream transit times and can be evaluated to obtain the flow velocity and the sonic velocity. If the transducer on the transmission side in such an apparatus is energized at the instant of transmission, it will not immediately emit the high frequency ultrasonic signal, which may for example have a frequency of 1 MHz, at maximum amplitude; the amplitude rather increases gradually. As a result, the end of the transit time, that is to say the commencement of the ultrasonic signal on the receiver side, can only be determined by the fact that the transducer on the receiver side receives an ultrasonic oscillation which is only little different from zero. This does not permit accurate measurement. In the known cases, therefore, the actual instant of comparison has been delayed with reference to the commencement of the signal. This occurs on the receiver side with the aid of arrival delay means by which the received ultrasonic signal is rectified, at least the first portion of the envelope of the rectified ultrasonic signal is converted to a substantially linear rising signal, and the latter is fed to a threshold value detector which, on reaching the threshold value, delivers an arrival delay signal that is delayed relatively to the actual arrival. In a second channel the received ultrasonic signal is amplified and clipped so that a rectangular signal of the same phase is obtained. The zero crossover of this rectangular signal with rising or positive-going leading edge following the arrival delay signal is utilized as a corrected arrival signal. Simultaneously, in the reference delay means a reference delay time was added at the instant of the actual reference signal, the reference delay time being constant, as is the arrival delay time, but somewhat larger than same. At the end of this reference delay time a time comparison signal is delivered as a corrected reference signal which is compared with the aforementioned zero crossover. It is immaterial whether on the receiver side the commencement of the arriving ultrasonic signal is determined very accurately. This is because a slightly retarded determination of the commencement of the signal merely displaces the arrival delay time and thus the arrival delay signal but not the selected zero crossover that is used for the measurement. This results in a very high measuring accuracy.
However, difficulties arise when the commencement of the arriving ultrasonic signal is determined so late that the arrival delay signal appears immediately prior to the selected zero crossover. In that case it can happen that the next zero crossover is instead used for the measurement and the measuring result is in error. One was therefore compelled to use very accurately operating components on the receiver side for permitting determimation of commencement of the signal as rapidly as possible, preferably still within the first half period.
The invention is based on the object of providing an improved apparatus of the aforementioned kind in which accurate determination of the commencement time is possible without placing such high requirements as hitherto on the accurate determination of the commencement of the arriving ultrasonic signal.